There has been known in the art a hydraulic circuit for supplying a discharge pressure fluid of a hydraulic pump into a hydraulic motor, in which the discharge pressure fluid of the hydraulic pump is supplied via an operating valve into one of a pair of principal circuits, of which a first is connected to a first port of the hydraulic motor and a second is connected to a second port of the hydraulic motor.
In a hydraulic circuit of this type, it is customary to use a safety valve of modulation type that is designed to make a relief of a portion of fluid of pressure elevated in the first and second principal circuits to ensure that the elevated pressure may not exceed a pre-established pressure, so that when the hydraulic motor is either to start or to stop driving, fluid pressure in the first or second principal circuit which is active may slowly be elevated in order to diminish a shock that can then be disadvantageously brought about therein. Such a safety valve as known in the art, that is capable of performing a modulation operation in which fluid pressure may rapidly be elevated up to a modulation start pressure and after then slowly be elevated up to a preset pressure, is shown in FIG. 1 of the drawings attached hereto.
More specifically, the safety valve has a valve seat 2 slidably inserted in a sleeve 1 in which a poppet 3 is also slidably inserted so that a concave conical surface 5 of the poppet 3 may be thrusted by a spring 4 against a convex conical surface 6 of the valve seat 2 to provide a valve interface. A piston 7 is slidably inserted in the poppet 3 to form a chamber 8 which is in fluid communication via a small opening 9 and via an inner bore 10 of the valve seat 2 with a high pressure port 11 whereas a low pressure port 12 is formed in the sleeve 1.
And, the sleeve 1 is formed at an end thereof with a large diameter section 20 whose open end has a plug 21 securely fitted therewith. The valve seat 2 is shaped to provide a stepped peripheral configuration having a small diameter one end portion 22, a large diameter mid portion 23 and an intermediate diameter opposite end portion 24. The small diameter one end portion 22 is inserted in the plug 21 to fit with an inner surface 21a thereof and is then sealed with a sealing material 25, whereas the large diameter mid portion 23 is inserted in the sleeve 1 to fit with the large diameter section 20 thereof and is then sealed with a sealing material 26. And, the intermediate diameter opposite end portion 24 is inserted in the sleeve 1 to fit with its inner surface 1a. Thus, the valve seat 2 is allowed to axially be slid and be displaceable by a distance L. Besides, the large diameter mid portion 23 and the sleeve 1 are shaped to provide an annular space 27 between the large diameter mid portion 23 and a step portion 1b of the sleeve 1. The annular space 27 is opening to the low pressure port 12 via an interstice 28 between the sleeve inner surface 1a and the intermediate opposite end portion 24, which is here designed to form a damper chamber A.
Let it be assumed that the section in which the valve surface 5 and the seating surface 6 are to be in contact with each other has a diameter d1, the piston 7 has a diameter d2, and the small diameter one end portion 22 of the valve seat 2 has a diameter d3.
An explanation is now given below of an operation of the conventional safety valve whose construction has been described above.
Assuming that the high pressure port 11 has a pressure that ranges between P1 and 0, the spring 4 forces the poppet 3 which in turn pushes the valve seat 2 to cause it to move by the distance L1 leftwards in the Figure to contact with the plug 21. Here, it should be noted that the spring load F1 of the spring 4 is set at a value that is lower than a conventional value by L1.times.K where K is a spring constant.
Under the state described, if the pressure P1 in the high pressure port 11 is suddenly elevated, the poppet 3 starts sliding rightwards when a thrust force due to the pressure P1 acting on the pressure receiving surface of the poppet 3 with an area A1=.pi./4.multidot.(d.sub.1.sup.2 -d.sub.2.sup.2) is balanced with the spring load F0 and then causes a section between the valve surface 5 and the seating surface 6, that is, the valve interface to open, thereby permitting a fluid of the elevated pressure to commence being admitted into the low pressure port 12, thus relieving the high pressure port 11. The pressure at this instant is a modulation start pressure.
Then, with the elevated fluid pressure P1 acting on the end surface 2a of an area A2=.pi./4.multidot.(d.sub.3.sup.2 -d.sub.1.sup.2) of the valve seat 2, the valve seat 2 is placed under a thrust force=A2.times.P1 to tend to move rightwards. However, also under the action of the damper chamber A, that is, the action in which pressure fluid in the annular space 27 is restricted in its flow by the interstice 28, flowing out of it and gradually into the low pressure port 12, the valve seat 2 must be more slow to move rightwards than the poppet 3. As a consequence, the pressure rises with a reduced slope towards arriving at a preset pressure when the seating surface 6 is urged to contact the valve surface 5.
The pressure of fluid in the high pressure port 11 under such a relief action will thus be to assume a waveform as represented by the solid curve shown in FIG. 2, rising in two stages with a long time elapsed until the preset pressure is reached.
With such a safety valve, it is therefore seen that a drop of pressure in the high pressure port 11 while it is under a relief action causes the poppet 3 and the valve seat 2 to move leftwards with the spring 4, returning to their initial positions. As the valve seat 2 is moved leftwards, fluid in the low pressure port 12 is forced to flow via the interstice 28, sucked into the damper chamber A.
However, since the interstice 28 that must be small and minimum in order to retard the rightward movement of the valve seat 2 impedes the fluid flowing that is sucking into the damper chamber A, the fluid fails to be sucked into the damper chamber A to an extent that is proportionate to the rate at which the valve seat 2 is moving leftwards via the poppet 3 with the spring 4. A negative pressure then is caused in the damper chamber A and gives rise to the formation of air bubbles therein.
The rightward movement of the valve seat 2 produced by rising of pressure again in the high pressure port 11 while air bubbles are formed in the damper chamber A causes the air bubbles to tend to be broken in the damper chamber A, allowing the valve seat 2 to be displaced rightwards quicker than at a rate that is solely governed by the flow restrictive action of the interstice 28. It follows then that a desired length of the time of modulation (i. e., the time elapsed from the instant at which the pressure is at a modulation start pressure until an instant at which the pressure reaches a preset pressure) cannot be achieved.
It can also be seen that since the damper chamber A is in fluid communication with the low pressure port 12 via the interstice 28 between the inner surface 1a of the sleeve 1 and the intermediate diameter opposite end portion 24 of the valve seat 2, a change in the diameter of either of the sleeve 1 and the valve seat 2 as caused by a change in temperature alters the size of the interstice 28. The size of the interstice 28 also fluctuates when the sleeve 1 or the valve seat 2 is eccentric. Such a change or fluctuation in the size of the interstice 28 bars a stabilized time span of modulation from being obtained.
As observed in the foregoing description, it has hitherto been recognized in the art to be difficult to adjust the time of modulation to be sufficiently long while the interstice 28 is kept minimum, which in turn makes it difficult to significantly diminish a shock that is entailed in a hydraulic motor when it is to start or end driving.
It is accordingly an object of the present invention to provide a safety valve which can overcome the problems mentioned above.